這裡參閱了網路上一段著名的對Servlet執行緒安全性進行測試的程式碼(http://zwchen.iteye.com/blog/91088)。執行之後，我們可以看一下這個例子的輸出：

package p.test.servlet.threadSecurity;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet(asyncSupported = true, urlPatterns = { "/threadSafeTestServlet" })
public class ThreadSafeTestServlet extends HttpServlet {

	private static final long serialVersionUID = -5360278358684235455L;
	// 定義一個例項變數，並非一個執行緒安全的變數
	private int counter = 0;

	public void doGet(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		doPost(req, resp);
	}

	public void doPost(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		// 輸出當前Servlet的資訊以及當前執行緒的資訊
		System.out.println(this + ":" + Thread.currentThread()
				+ " Startttttttttttttttttttttt");
		// 迴圈，並增加例項變數counter的值
		for (int i = 0; i < 5; i++) {
			System.out.println(this + ":" + Thread.currentThread()
					+ "Counter = " + counter);
			try {
				Thread.sleep(20000);
				counter++;
			} catch (InterruptedException exc) {
			}
		}

		System.out.println(this + ":" + Thread.currentThread()
				+ " Enddddddddddddddddddddddddd");

//		resp.getWriter().println(
//				this + ":" + Thread.currentThread()
//						+ " Enddddddddddddddddddddddddd");
//		resp.getWriter().println("Done");
//		resp.getWriter().flush();
//		resp.getWriter().close();

	}
}
 

[email protected]:Thread[http-8081-Processor23,5,main] 
Counter = 60 
Counter = 61 
Counter = 62 
Counter = 65 
Counter = 68 
Counter = 71 
Counter = 74 
Counter = 77 
Counter = 80 
Counter = 83 


[email protected]:Thread[http-8081-Processor22,5,main] 
Counter = 61 
Counter = 63 
Counter = 66 
Counter = 69 
Counter = 72 
Counter = 75 
Counter = 78 
Counter = 81 
Counter = 84 
Counter = 87 

Tomcat 3.0, 預設的狀態下，沒有大量請求時，併發數好像是 50，

更大量的請求到來時，預設的併發數好像可以自動增加到 300，

但在這裡，像這樣如果命名一個 類變數 counter，肯定不是執行緒安全的，